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ABSTRACTa period of low, above-freezing temperatures, termed cold-hardening. Cold-hardening is a critical event in theThe survival of cereal crops during winter depends primarily on the over-wintering physiology of plants (Levitt, 1980 with an increase in freezing tolerance (Steponkus, 1978).stages of recovery after they had been grown and frozen under controlled conditions. Our results confirmed those reported for barleyHowever, it has been difficult to establish cause and

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Significant increases in the production of winter cereals could be realized with improved hardiness. The objective of this research was to determine if overwintering cereals modify water soluble carbohydrate composition in response to naturally occurring winter freeze stress. Fields of wheat (Triticum aestivum L.), barley (Hordeum vulgare L.), and rye (Secale cereale L.) were monitored throughout the winter between 1986 to 1990 for snow-pattern development, temperature, and snowfall. Plant crowns were analyzed for freeze injury (form and intensity) and water soluble carbohydrate composition (froctan, sucrose, glucose, and fnlctose). Growth chamber plants were frozen and analyzed to confirm the interpretation of field plant data. In 1986, there was less froctan and more soluble sugars in the exposed than in the snow-covered wheat plants (but no complete depletion of fructan), and no crown injury from equilibrium freezing. In 1987· 1988, little injury or froctan hydrolysis occurred in barley, wheat, or rye; the plants were snow covered during cold periods. In -1990, fructan hydrolysis to sugar in exposed barley, wheat, and rye plants paralleled that in the exposed wheat plants of [1986][1987]. The fructan content of exposed barley plants was nearly depleted during the coldest weather. Exposed plants had typical equilibrium freeze injury. Growth chamber plants responded similarly to the exposed field plants. Plants exposed to freeze stress converted fructan to sugars which probably alleviated adhesive freeze stress. The energy of hydrolysis may be useful for placing sugar into sites within the tissue where its cryoprotective activity is most effective.

An increase in intercellular solute of crown tissue was induced by keeping hardened plants frozen at −3°C for 24 h. This environmental condition commonly occurs because of temperature stabilization from latent heat released as soil water freezes. The amount of intercellular solute in crown tissues was estimated from the concentration of liquid that, when perfused through the plant crown at 1°C, was found, by successive approximations, to be isotonic with the intercellular liquid. The intercellular content of ‘Rosen’ rye (Secale cereale L.) increased by factor of 3.0 to 3.5 when frozen 24 h at −3°C; then at 1°C, the concentration gradually decreased to that of nonfrozen plants. Recovery occurred in less than 1 h at 25°C. The increase of intercellular solute content occurred as a function of time at −3°C. ‘Hudson’ barley (Hordeum vulgare L.) which contains less intercellular solute than Rosen rye after freezing at −3°C, and Rosen rye in which the intercellular solute was reduced by flushing, were less hardy than Rosen rye tested with the normal 24‐h prefreeze at −3°C.

In freezing, competitive interaction between ice and hydrophilic plant substances causes an energy of adhesion to develop through the intersitial liquid. The thermodynamic basis for the adhesion energy is disussed, with estimates of the energies involved. In this research, effects of adhesion energy were observed microscopicaUly in conjunction with energies of crystaflization and frost desication. The complex character of ice in intact crown tissue of winter barley (Hordeum vulgare L.) and the problems of sectioning frozen tissue without producing artifacts led to an alternative study of single barley ceUs in a mesh of ice and cell wall polymers. Adhesions between ice, cell wall polymers, and the plasmalemma form a complexly interacting system in which the pattern of crystallization is a major factor in determination of stress and injury.Freezing of water within crown tissues of cereals causes several different forms of stress energy to develop. These stresses have been distinguished by differences in the dissipation of crystallization energy, the temperature range in which the stress becomes injurious, and the histological pattern of injury in the plant (10). The cause of injury in the temperature range between -8 and -16 C has been the most elusive. Direct injury from growth of ice crystals requires a large free energy of crystallization. This can only develop from supercooling or from rapid transfer of latent heat in very wet tissue (6), and therefore is most often effective in winter cereals, above -8 C. Frost desiccation, where ice acts independently through the vapor phase as a water accumulator, does not injure leaf tissues of hardened "Hudson" barley (Hordeum vulgare L.) until the ice temperature is below -16 C (7).A thermodynamic study of equilibrium freezing in complex interfaces between ice and hydrophilic substances led to the conclusion that competition for the interfacial liquid water caused an energy of adhesion to develop (8-10). The adhesion energy developed through competitive structuring of the interfacial liquid and was predicted to cause significant stress in the temperature range near -10 C.Effects of adhesion energy were observed microscopically in conjunction with effects of crystallization and frost desiccation. The complex character of ice in intact crown tissue and the problems of sectioning frozen tissue without producing artifacts led to an alternative study of single winter barley cells in a mesh of ice and cell wall polymers. (Fig. 1). The heat sink and thermal contact used for testing cells were the same as those used for testing leaves in order to generate equivalent crystallization energies in the intercellular liquid for a specified degree of supercooling in the nonequilibrium freeze test ( Fig. 1) MATERIALS AND METHODS

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